Modulation



March 28, 1,939. M BAESECKE ET AL 2,152,016

MODULATION Filed Aug. l5, 1935 -gin- CHOKE D Patented Mar. 28, 1939 rMO'DULAT'ION Martin Baesecke and Rudolf Bauer, Berlin, Germany, assignors to Siemens und Halske Aktiengesellschaft, Berlin, Germany, a corporation of Germany Application August 15, 1935, Serial No. 36,424 In Germany'Augllst 18, 1934 5 Claims.

This invention relates to a new and improved means for modulating one alternating current in accordance with another. The current modulated may be the origin of the modulation current.

'Ihe use of voltage-dependent resistances for the modulation of alternating current is known in the art. For instance, instead of the formerly customary tube modulators, -modulators have been made from cuprous oxide or similar materials. While presenting several advantages, modulators of this kind have the drawback that they introduce an additional damping in the transmission path,and this damping must inthe majority of cases be compensated again by the inclusion of additional amplifiers in the line. In other words, the use of the said modulators of the oxide or similar type does not makeit possible to dispense with tubes and to obviate such inconvenience as is attended with the use of tubes. However, in a great many cases it is especially desirable to avoid the use of tubes. For instance, a carrier-current telephony system for rough handling and operation subject to frequent changes in location will only be feasible readily and work under stable conditions if it does not involve sensitive yvacuum tubes and heavy batteries.

These conditions cited by way of example for a purpose as stated are fulfilled according to this invention by using for modulation one or more voltage-dependent resistances, the operating points of said resistances lbeing soA chosen that they operate on a drooping part of their current voltage characteristic curves.-In this instance, the introduction of the modulator elements in the circuits does not mean an increase in attenuation of the signal therein, but, on the contrary causes a reduction -in damping in the transmission circuit so that additional boosting yamplifiers may be dispensed with. Any substance known in the art having negative resistance characteristics may be used. As example of resistances which may be used, reference isv made to United States Patents #896,240, #1,019,390, #1,693,825, and #1,954,832. f'

The adjustment of the working point may be accomplished by' any desired ways and means, e. g. theresistance as known in the art may be provided with vsuitable voltage or current bias, or their temperature may be acted upon, etc.

In the drawingwe have shown severalexempliiied embodiments of the invention. In Figure 1, we have shown a. balanced modulator of the carrier suppressor type. In Figure 2-we have shown a frequency divider in which the submultiple frequency acts on the base frequency to enhance the frequency division effect. In Figure 3V we have shown graphically the volt-ampere characteristics of resistive material having the desired characteristics; while in Figure 4 we have shown a modification of the arrangement in Figure 1. In the arrangement of Figure 4 additional non-linear resistances are connectedin circuit. I'hese additional resistances suppress or eliminate additional frequencies, Where desired, from the output. x

Referring to Fig. 1, I denotes an audio frequency source, such as a microphone of a telephone subscribers omce or instrument. 'I'he voice frequency is to modulate the carrier frequency furnished from generator 2, and one of the side-bands resulting from such modulation is to be put on the telephone (or long-distance) line 3. In a bridge arrangement of the kind known in the earlier art which suppresses the carrier frequency we provide two voltage-dependent resistances 4 and 5 which are as near as possible of like characteristic. The resistances are operated in such a manner that between l and the input to filter 6 which serves to lter out the desired side-band there will be no or substantially no attenuation of the wave energy, but, on the contrary, there will be an amplification of the transmitted energy. The modulator described briefly herein is of the type disclosed in U. S. Patent #1,343,306.

The non-linear resistances 4 and 5 may be of any appropriate type as long as each has the characteristic illustrated in Figure 3. This illustration brings out this fact that, with growth of current, the potential of the resistance initially rises inside the range marked I-2, and that it decreases again inside the ranges marked 2, 3, 4, 5, 6, as the current grows still further. The optimum working point lies around the point marked 4 to which is coordinated the current Ic. This bias load current is furnished by a direct current. Upon this direct current according to Figure 3, alternating currents are superimposed. The maximum amplitudes of the alternating current fall approximately inside the range between 3 and 5.

As an example showing how the resistances 4 and 5`may be fed with the direct current In, we have shown in Figure 1 in parallel relation to each of the negative resistances a circuit which comprises two choke-coils D, a regulating resistance W, and a direct current battery B. The chokes D are provided for the purpose of preventing short-circuiting'of the alternating currents by Way of the battery. The regulating resistance W is for the purpose of adjusting the direct current Io to the proper value. To prevent the directl current of the battery from acting also upon the other circuits, the negative resistance is blocked by condensers e at both ends. The condensers have been chosen so high that they will oier practically no resistance to the fiow of the alternating current. The arrangement such as illustrated in Figure 1 holds good for all non-linear resistances operated as negative resistances as disclosed herein, for instance, both for the resistance 4 as well as the resistance 5 in Figure 1 and resistances 'l and 8 of Figure 4 described hereinafter.

'I'he arrangement of these resistances, as to the rest, may be varied at will. For instance, if the carrier frequency is not to be eliminated, then one voltage-dependent resistance will suffice. If in addition elimination of the modulating voltage is demanded, a bridge comprising four resistances may be employed as known in the art. (See French Patent #739,103, Jan. 5, 1933). The two additional resistances 'l and 8 forming this bridge of 4 arms have been shown in Figure 4. Each of these non-linear resistances as Well as nonlinear resistances 4 and 5 are operated as described in connection with Figure l. Each resistance is biased by direct current applied in any convenient manner, for example, as shown by means of a source B and a variable resistance W connected `in shunt to each non-linear resistance by means of choke coils D.

It is known that by the aid of modulation actions it is possible to derive from one frequency another frequency `being a multiple or a submultiple of the original frequency. Also for these frequency transformer circuit schemes, modulators operated in accordance with this invention may be used advantageously. One exemplied embodiment illustrating this case is shown in Fig. 2. Suppose the problem arises to step down the current furnished from source I, of, say 1000 cycles per second at the ratio of 1:4. For this purpose, a circuit is connected in parallel relation to the voltage-dependent resistance 2 biased as described above to operate Aon the drooping branch of the current-voltage characteristic as illustratedtin Figure 3, the resistance (impedance) of said 'circuit being low in the neighborhood of the desired frequency. In Fig. 2, there is indicated by Way of example the circuit 3 tuned to 250 cycles per second. From the current source l there is impressed upon `the voltage-dependent resistance a current of frequency 1000 cycles per second. By an impact applied from the outside, say, by the circuit closing rush, a nearly continuous frequency spectrum is produced which comprises inter alia the desired frequency of 250 cycles per second. This frequency is preferentially treated `or emphasized .in the said tuned circuit 3 and as a consequence it will arise at a substantially increased amplitude across the terminals of 2. By modulation with the frequency of 1000 cycles per second, there arises rst a frequency of 750 cycles per second and nally, again by modulation of the latter frequency of '750 cycles per second with the original current of 1000 cycles per second the desired frequency of 250 cycles per second. The amplitude of the desired frequency of 250 cycles per second may be increased by the connection of a circuit Vin parallel relation to 3 and tuned to 750 cycles per second. The stabilization of the frequency ratio is here insured lby the modulation products of higher order; for instance, the frequency to be obtained and the modulation product of the third order of 1000 and 250 cycles per second, i. e. 1000-3 X250 cycles per per second are equal, i. e. 250 cycles per second. The desired frequency can be obtained across one or several elements of circuit 3. In the same manner `any other desired whole-number (integral) frequency relations may be secured.

What is claimed is:

1. In a frequency changing circuit, a nonlinear resistance of conductive substance, having a drooping voltage-ampere characteristic when properly biased by vdirect current, a circuit including a source of direct current connected with said resistance vfor biasing said non-linear resistance for operation on the drooping portion of its characteristic, a source of alternating current connected with said non-linear resistance, and a reactive circuit connected in parallel with said resistance,said circuit being tuned to a frequency related to the frequency of said source.

2. Ina :modulation system of the carrier suppressor type, a pair `of impedances having characteristics such that when properly polarized by direct current an increase of current flow through the impedances -results in a decrease in potential drop across the impedances, a source of direct current connected with each of said impedances for polarizing the same to operate on the drooping portion of their characteristics, a resistance in Veach of said connections for stabilizing the operation -of `said impedances on said drooping portions of their characteristics, an alternating currentl circuit connected to one terminal of each of said impedances, means for impressing modulating potentials on ysaid alternating current circuit, an alternating current lcircuit, connected between the remaining terminals of said impedances, an output 'circuit coupled to said last named circuitfand Aa source of currents to be modulated connected to points of symmetry on said first named'circuit and on said second named circuit.

3. In Va modulation .system vof the type wherein the modulated `and modulating currents are suppressed, a plurality of impedances each having a drooping characteristic vsuch that when properly biased `by direct `current an increase of current through the impedances results in a decrease in the potential drop through the impedances, -a direct current circuit connected with each of saidimpedancesfor operating the same on the drooping portion of its characteristic, a resistance in each of said direct current circuits for stabilizing vthe operationof Asaid impedances on the drooping portions of their characteristics, a first alternating `current circuit connected between .a pair of said impedances, means for impressing :modulating currents on said circuit, a second alternating `current lcircuit connected between the free terminals `of said pair of impedances, a load circuit coupled to said second circuit, means for impressing currents to be modulated on symmetrical points on said first and second circuits, and circuits connected other of said impedances between opposed terminals of said pair of impedances.

4. In a. modulationsystem, a source of modulating current, a source -of current to be modulated, an impedance of the substantially solid conductor type having a drooping lcharacteristic such that .an increase of direct current through the impedance results in a decrease of potential drop across the impedance, means for biasing said impedance to operate on the drooping portion of its characteristic and for stabilizing its operation on said drooping portion comprising a direct current circuit including a source of direct current and a resistance connected with said impedance, said direct current circuit supplying current to said impedance of an intensity such that it operates on the drooping portion of its characteristic, said resistance stabilizing the operation on said drooping portion of the characteristic, means for applying current from both of said sources to said impedance and an alternating current output circuit coupled to said irnpedance.

5. In a device for producing alternating currents of a frequency related to the frequency of a source of alternating current, a rst resistance of the substantially solid conductor type having a drooping voltage ampere characteristic When properly polarized by direct current, a circuit including a second resistance and a source of direct current connected With said first resistance for biasing the operation of said first resistance on the drooping portion of its characteristic and substantially stabilizing the operation of said first resistance on said portion, a circuit for applying alternating current from said source to said rst resistance, means for applying alternating current of a different frequency to said first resistance, and an output circuit responsive to a frequency which is related to the frequency of the alternating current of said source coupled to said first resistance.

MARTIN BAESECKE'. RUDOLF' BAUER. 

